1. Field of the Invention
The invention relates to a projection objective for microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits and other microstructured components. The invention relates in particular to projection objectives in which at least one optical element can be deformed with the aid of an actuator. The invention furthermore relates to a method for correcting optical properties of such a projection objective.
2. Description of the Prior Art
The purpose of projection objectives in microlithographic projection exposure apparatus is to project a reduced image of structures contained in a mask onto a photosensitive layer, which is applied on a support. Very stringent requirements are placed on the imaging properties of the projection objectives owing to the small size of the structures to be image.
U.S. Pat. No. 6,388,823 assigned to the Applicant discloses a projection objective in which forces can be exerted on an optical element with the aid of actuators, which are distributed around the circumference of the optical element. These forces lead to bending of the optical element which, for example, may be a lens or a mirror. The bending does not affect the thickness profile of the optical element, or at least does not substantially affect it. In particular, astigmatic imaging errors can be corrected with such a bendable optical element.
It has however been found that the bending is not only accompanied by the desired corrective effect; rather, undesired impairment of the imaging properties of the projection objective can also occur. This applies in particular to optical elements which are isostatically mounted. Isostatic mounting means that the optical element is held merely at three mounting points offset by 120° along the circumference.
Such undesired impairment of the imaging properties, however, may also be caused by optical elements which are not bent but otherwise deformed in order to correct imaging errors, as is known per se in the prior art.